#!/bin/bash
a="abc"
b="def"

echo "a='abc' b='def'"

if [ $a = $b ]
then
  echo "a=b:true"
else
  echo "a=b:false"
fi 

if [ $a != $b ]
then
    echo "a!=b:true"
else
    echo "a!=b:false"
fi

if [ -z $a ]
then 
   echo "a=0:true"
else
   echo "a=0:false"
fi

if [ -n $a ]
then 
   echo "a!=0:true"
else
   echo "a!=0:false"
fi

if [ $a ]
then 
   echo "a!=null:true"
else
   echo "a!=null:false"
fi